1. Field of the Invention
This invention relates to gaseous sterilization which is the treatment of objects or materials with a chemical in the gaseous or vapor state to destroy all microorganisms with which they have contaminated. The need for such a method of sterilization has developed from the use of many items that cannot be subjected to heat, radiation, or liquid chemical sterilization.
2. Description of the Prior Art
In practice, only two gases or vapors have been commercially used on a large scale for surface sterilizing purposes; they are formaldehyde vapors and ethylene oxides gas.
Formaldehyde vapors have been used as a fumigant for many decades in the Hospital, agricultural and industrial fields. The limitations of this technique are numerous. To kill tough aerobic and anaerobic bacterial spores at room temperature, one needs at least a 24 hour contact time with a vapor having at least 70% relative humidity. This type of vapor is extremely corrosive and the fumes are very irritating. It is also very difficult to maintain a high level of formaldehyde gas since CH.sub.2 O is stable in high concentrations only at temperature above 80.degree. C. in humid air. At ordinary room temperatures formaldehyde gas quickly polymerizes and it dissolves readily in the presence of water. Thus gaseous sterilization with formaldehyde has been regarded as a misnomer because introduction of formaldehyde gas into a closed space serves mainly as a mechanism for distributing either moisture films in which formaldehyde is dissolved or solid formaldehyde polymers over all available surfaces within the enclosed space. This indeed explains why very inconsistant and sometimes contradictory results have been reported in hospital disinfection, patient rooms, bedding, etc., or agricultural applications such as eggs and hatcheries sanitizing. Formaldehyde vapor has very weak penetrating ability and, if used in an atmosphere with traces of hydrochloric acid, it can quickly produce (70.degree. F., 40% RH) Bis-(chloromethyl)-ether which is a carcinogenic agent.
To minimize the abovementioned drawbacks in Hospital applications, a new approach was recently developed which combines the use of subatmospheric steam and formaldehyde gas at 80.degree. C. in autoclaves. This method is said to kill most sporulated microorganisms at the concentration normally encountered in Hospital practice while decreasing the aldehydes residue on instruments. It requires a time exposure of two hours with a formalin concentration of 8 gr. per cubic foot of autoclave. However, despite the long contact time and the relatively high temperature, the method would not satisfy today's stringent requirements of the sporicidal AOAC test in this country.
In short, formaldehyde vapors, besides their toxicity and irritating characteristics are difficult to handle at room temperatures and they do not provide a fast and reliable method to satisfactorily handle most of the Hospital and industrial applications.
This may explain why in the past two decades ethylene oxide (ETO) has become the most popular method to gas sterilize both in Hospitals and industry. In a recent survey entitled "Use of Ethylene Oxide as a Sterilant in Medical Facilities" (NIOSH, August, 1977), Dr. Z. R. Glaser mentioned that at least 6,500 hospitals use ETO sterilizers. Since most average hospitals (i.e., 200-300 beds) use at least two units, it is estimated that at least 20,000 hospital ETO sterilizers are in use. This corresponds to an investment of the order of 120 million dollars with replacements and sales of new units growing at a fast rate.
While at the beginning ETO seemed an ideal technique to replace formaldehyde fumigants, very serious limitations from the toxicity view point recently attracted the attention of Health Authorities.
In the previously mentioned NIOSH report, it is stated that
"The acute toxic effects of ETO in man and animals include acute respiratory and eye irritation, skin sensitization, vomiting and diarrhea. Known chronic effects consist of respiratory irritation and secondary respiratory infection, anemia, and altered behavior. PA1 "The observations of (a) heritable alterations in at least 13 different lower biological species following exposure to ETO, (b) alterations in the structure of the genetic material in somatic cells of the rat, and (c) covalent chemical bonding between ETO and DNA support the conclusion that continuous occupational exposure to significant concentrations of ETO may induce an increase in the frequency of mutations in human populations At present, however, a substantive basis for quantitative evaluation of the genetic risk to exposed human populations does not exist . . . However, the alkylating and mutagenic properties of ETO are sufficient bases for concern about its potential carcinogenicity. Neither animal nor human data are available on which to assess the potential teratogenicity of ETO . . . Although this review is limited to ETO, concern is also expressed for hazards from such hydration and reaction products of ETO as ethylene glycol and ethylene chlorohydrin, the latter a teratogen to some lower biological species."
The average time needed to sterilize medical instruments in an ETO unit is 180 minutes at 30.degree. C., but it has to be followed by a long de-aeration period. For instance, the de-aeration time for medical devices is comprised between 2 and 8 hours in a de-aerator machine, but it oscillates between 1 and 8 days at room temperature. On rubber gloves, the residues can burn the hands; on tubes carrying blood, they will damage red blood cells and cause hemolysis. Endotracheal tubes which are not properly aerated can cause tracheitis or tissue necrosis.
Besides the risks due to the toxicity of ETO residues, other accidents have been reported due to the explosive characteristics of pure ETO. As little as 3% ethylene oxide vapor in air will support combustion and will have explosive violence if confined. To solve this problem, various gases such as CO.sub.2 or fluorinated hydrocarbons have been mixed with ETO in some commercial formulations.
In short, ETO sterilization grew tremendously not because it was an ideal solution, but rather because there seemed to be no alternative gas sterilant method which was capable of as fast a sporicidal action without any drawbacks from the toxicological or environmental view point.
The object of the present invention is to provide an alternative to ETO sterilization with the advantages of faster sporicidal action, no de-aeration period, no toxic residue, and no explosion risk. Moreover, this invention will provide a more economical approach from the running and investment cost view point when comparing the volume of material treated per unit of time.
When speaking hereafter of sterilization, it always refers to sporicidal action against B. subtilis ATCC 19659 and Clostridrium sporogenes ATCC 3584 because they are the resistant microorganisms used in the fumigant-sterilant test according to the legal requirements of the AOAC (Official Method of Analysis of the Association of Official Analytical Chemists, 12th ed., Nov. 1975). It is indeed to be understood that the destruction of these two species of spores according to the official procedure means automatically the destruction of other less resistant microorgaisms such as Mycobacteria, non lipid and small viruses, lipid and medium size viruses and vegetative bacteria.